The present invention relates to a roller track for guiding an ingot from the bottom opening of a mold for continuous casting, along a curved path for continued withdrawing in the horizontal.
The ingot exiting from a continuous casting mold has a just barely solidified outer shell or skin and is still liquidous in its interior. The skin is not only thin at first but the liquid core exerts a ferrostatic pressure tending to bulge the skin, which, of course, may rupture. Thus, the ingot must be carefully supported until solidified completely.
It is customary to support the ingot by means of roller tracks engaging the ingot, e.g. from above as well as from below; that is to say, on the inside as well as on the outside of the curved withdrawal path. Still some bulging is inevitable and depends on the distance from roller to roller, on the skin temperature and thickness, and on the ferrostatic pressure. The latter depends on the height of the machine because the molten metal forms a liquid column that reaches down from the surface level of the molten metal in the mold to the lowest horizontal level of still liquidous metal in the core of the continuous cast ingot. One has only a limited degree of control here; reducing the height of the column requires a sharper curving of the withdrawal path, but obviously there are limitations present here; the curving must be rather gradual.
The skin temperature can and will be reduced by so-called secondary cooling, e.g. vigorous spray cooling the ingot from the outside as it exits from the mold and over a long portion of the withdrawal path. However, some grades of steel are not amenable to very strong cooling from the outside. Spraying water onto the emerging ingot amounts to a local quenching and may cause formation of cracks. In other words, steel requires a more gradual cooling, but this delay inherently increases the tendency to bulge. On the other hand, it should be noted that due to the geometry involved, one can apply more cooling water to the underside (outward curving) than to the upper side without producing larger fissures.
The withdrawal rollers should be placed next to each other as close as possible, but clearly there are limitations here resulting from the physical dimensions of the rollers. Of course, the main point is to decrease the distance between points of support, and that can be achieved by making the rollers smaller. However, if the rollers are too thin, they bend of their own accord. One has used, therefore, split rollers, i.e. rollers of axially short dimensions. However, the support of such rollers was found to be quite different and complex, particularly if used throughout both roller track portions.
The known roller tracks have usually similar rollers and a similar number of rollers on the inside -- upper side as well as on the outside -- underside. The distances are slightly different due to the curvature of the track, but the track curves quite gradually and the number or rolls is usually quite large, so that that difference is insignificant.